A variety of RFID systems exist. They differ in terms of range, size, cost and underlying technology and can be classified in two main classes operating in different frequency bands: inductive systems operating at frequencies lower than 100 MHz and radiative systems operating at frequencies greater than 100 MHz.
The difference between the two classes is based on the type of the physical coupling between the reader and the RFID tag which could be either magnetic (inductive coupling) or electromagnetic (radiative coupling). For the first class of RFID technology, the RFID tag gets its energy from the proximity coupled magnetic field and responds by loading its own antenna with different impedances, while for the second class, the RFID tag gets its energy from the electromagnetic field radiated by the reader and reflects it back modulating with its own impedances presenting different Radar Cross Section (RCS).
The coupling nature of the first class (inductive coupling) limits the read range to the size of the reader or the tag antenna (generally few centimeters) while the range of the second class-radiative coupling) could reach up to tens of meters depending on the nature of tags (passive and active) and its sensitivity. The popular access control application based on Near Field Communication (NFC) standard falls in the first class.
The present invention sets out to addresses the second class of long range RFID systems, namely the radiative RFID systems operating in the UHF and above frequency bands. The exact frequency bands and associated maximum allowable radiated power of currently available RFID systems are given in Table 1 and 2 below.
TABLE 1Relevant standards of RFID systemsFrequency range<135 KHz [LF]13.56 MHz [HF]860-960 MHz [UHF]2.45 GHz [Microwave]Relevant standardsISO 11784 & 11785ISO/IEC 18000-3ISO/IEC 18000-6ISO/IEC 18000-4ISO/IEC 18000-2EPC class-1EPC class-0, class-1ISO 14223-1ISO 15693ISO 14443 (A/B)
TABLE 2ITU Frequency regulationa. LF Band (119-135 kHz)USA/CanadaEuropeJapanChina2400/f(inkHz)mW/m @119-127 kHz:30 V/m @ 3 mPmwk < 1 W300 m66 dBμA/m @ 10 m127-135 kHz:42 dBμA/m @ 10 mb. HF Band (13.56 MHz)USA/CanadaEuropeJapanChina13.553-13.567 MHz13.553-13.567 MHz13.553-13.567 MHz13.553-13.567 MHz42 dBμA/m @ 10 m42 dBμA/m @ 10 m42 dBμA/m @ 10 m42 dBμA/m @ 10 mc. UHF Band (860-960 MHz)USA/CanadaEuropeJapanChina902-928 MHz865.0-868.0 MHz Pe.r.p. = +20 dBm952-955 MHz840.5-854.5 MHz Pe.r.p. = 2 WPe.i.r.p.2 = 4 W865.6-868.0 MHz Pe.r.p. = +27 dBmPe.r.p. = 1 W + 6 dB ant920.5-924.5 MHz Pe.r.p. = 2 W865.6-867.6 MHz Pe.r.p. = +33 dBmgain = 4 W(Available since May 2007)d. Microwave Band (2.45 GHz)USA/CanadaEuropeJapanChina2.400-2.483 GHz2.446-2.454 GHz2.400-2.4835 GHz 2.400-2.425 GHz 250 mW/m @Pe.i.r.p. = 4 WPe.i.r.p. = 500 mW or 4 W (Indoors)3 mW/MHz(Pe.i.r.p. = 1 W)3 m (Pe.i.r.p. = 21 mW)1Listen-before-talk for 200 kHz channels.2Equivalent isotropically radiated power (e.i.r.p) = 1.64 × Effective Radiated Power (e.r.p.)
The exact frequency bands and associated maximum allowable radiated power of currently available RFID systems are generally fixed by local and regional regulation bodies (ETSI, FCC etc.). Further to the frequency bands indicated in Tables 1 and 2, some RFID applications use the 433 MHz frequency band and the use of the 5 GHz ISM band is expected for the future for WiFi systems. Therefore, in order to insure the operability of RFID tags all over the world, it may be worth to have a tag that could work at more than one frequency.
On the other side, UHF frequency bands and microwave frequency bands suffer from range limitation and interferences due to the well-known multipath frequency selective fading. FIG. 1 shows a graph presenting an example of variations of magnitudes of a received RF signal at UHF frequency within an indoor area having coordinates X and Y of 2 m×2 m, respectively. As can be seen on this figure, variations of the RF signal of up to 40 dB could appear over distances of a few tens of centimeters. Such fast fading signal could, disadvantageously, prevent the activation of one RFID tag even for a transmission from a close RFID reader whereas the same tag could be disadvantageously activated by a distant RFID reader increasing the interference issue. A situation may happen in which an RFID tag is located in a so-called fade region with respect to a signal transmitted by a close RFID reader and in a so-called crest region with respect to a signal transmitted by a distant RFID reader.
To mitigate this well-known problem it is possible to use antenna diversity (or spatial diversity) at the reader or the tag side. The use of spatial diversity requires at least two antennas with a minimum of space separation between the two antennas. Typically a quarter wave lambda separation is required in order to insure sufficient de-correlation between the two antennas. This minimum distance between the two antennas is approximately equal to 8 cm at 900 MHz and 17 cm at 433 MHz.
This minimum distance is not always available in the reader side, in particular when the reader is integrated into mobile devices such as a smartphone or a tablet. The required space for antenna diversity is even less available in the tag side and many objects to be tagged are sized to receive only one single small antenna.